RE: User is able to see all tasks
Can you please check the security role of the user with whom you have shared the records? If the user doesn’t have read privilege of the activity entity, then they will not be able to see the activities on Timeline. Also, make sure that you have given at least read access permission while sharing the records.
Select record -> Share (Ribbon button) -> Select User/Team -> Permissions
When you have given the read access permission, you can see the activities on Timeline as shown in the below screenshot.
When you haven’t given the read access permission, you can’t see the activities on Timeline as shown below.
On the same screen, you can see with whom you have shared the records and what permissions are given.
Hope this helps.
Thanks!